An AC-DC transformation circuit as indicated in FIG. 4 is used as a multipath detection means in an FM tuner.
In the figure, T.sub.i represents an input terminal; T.sub.o indicates an output terminal; C.sub.1 .about.C.sub.4 are condensers; R.sub.6 .about.R.sub.11 are resistances; Q.sub.0 is a transistor; D.sub.1 .about.D.sub.3 are diodes; and +V.sub.cc and +V represent power supplies.
AMP indicates an amplifier circuit, which is constructed by a transistor Q.sub.0 and resistances R.sub.6 .about.R.sub.8, and on the other hand REC is a voltage doubler rectifier circuit, which consists of diodes D.sub.1, D.sub.2 and condensers C.sub.2, C.sub.3.
In the construction described above AC input signals (voltage) V.sub.i consisting of multipath signals etc. applied to the terminal T.sub.i are fed through a coupling condenser C.sub.1 to the amplifier circuit AMP, where they are amplified, and this amplified output is applied to the voltage doubler rectifier circuit REC. The voltage doubler type rectifier circuit is used in order to obtain a rectified output as large as possible and to increase the sensitivity to detect multipath signals. During positive half cycles the condenser C.sub.2 is charged through the diode D.sub.1 with the polarity indicated in the figure. In the same way, during negative half cycles the condenser C.sub.3 is charged through the diode D.sub.2 with the polarity indicated in the figure. In this way a rectified output, which is about twice as high as the amplified output, can be obtained between the two terminals of the condenser C.sub.3 with the polarity indicated in the figure.
Further a condenser C.sub.4 is charged by the power supply +V and when the diode D.sub.3 is cut off, the potential at its hot side (side opposite to ground) terminal is kept at +V. A diode D.sub.3 is used for level shift.
When the AC input signals V.sub.i applied to the above mentioned input terminal T.sub.i increase under the off state of the diode D.sub.3 ; the terminal potential of the condenser C.sub.3 in the voltage doubler rectifier circuit REC rises accordingly; when this value (negative voltage) exceeds the forward voltage V.sub.D of the diode D.sub.3, the diode D.sub.3 is turned on at this moment and at the same time electric charge in the condenser C.sub.4 is discharged through the diode D.sub.3.
As the result, the voltage +V is transformed into a DC voltage V.sub.0 depending on the level of the AC input signals V.sub.i applied to the terminal T.sub.i, which is outputted at the terminal T.sub.0.
When no AC input signals V.sub.i are applied, or when they are applied, but their rectified output is lower than the above mentioned forward voltage V.sub.D, since the diode D.sub.3 keeps its cut off state, the terminal voltage +V of the condenser C.sub.4 is outputted at the terminal T.sub.0 as a DC voltage, as it is.
In this way, when multipath signals are produced, the voltage of the power supply +V is transformed into a DC voltage V.sub.0 to be outputted, dependent on the level of the AC signals V.sub.i so that its level can be detected.
The prior art AC-DC transformation circuit constructed as described above has the following problematical points.
(i) The detection sensitivity cannot be raised.
In order to raise the detection sensitivity, it is necessary to transform surely the AC input signal V.sub.i into a DC output, even if its level is low. For this purpose it is necessary that the rectified output level, i.e. the amplifier output level is high. Consequently the gain of the amplifier circuit must be increased. However, a multi-stage amplification is necessary therefor and thus it is feared that not only the number of parts is increased, but also operation of the circuit becomes unstable.
(ii) It is a mean value that is detected.
In order to effect air accurate multipath detection, it is necessary to detect peak values of the input signals. However, in the case where a low level signal is produced instantaneously (singly), although the aforementioned condenser C.sub.3 is charged, its terminal potential cannot reach the value necessary to change the cut off state of the diode D.sub.3. Only when these signals are produced frequently, the level of each of the signals is integrated and the diode D.sub.3 can be turned on. The DC voltage detected in this case is an average value of a plurality of signals and doesn't correspond to each of the signals.
(iii) Response characteristics are not good.
Since a condenser is used therein, a certain charging time is necessary and its detection work is slow correspondingly. Although the response characteristics can be ameliorated by using a small capacity condenser, it decreases the effect to removing AC components and thus decreases the AC-DC transformation efficiency.
(iv) The operation starting point cannot be set arbitrarily.
Although a level shift of the operation starting point of the AC-DC transformation circuit by a voltage drop corresponding to the forward voltage V.sub.D of the diode D.sub.3 is possible, no level shift smaller than V.sub.D is possible.
(v) The circuit is not suitable to be made in the form of an IC.
Since a voltage doubler rectifier circuit is used therein,, large capacity condensers C.sub.2, C.sub.3 are necessary, which is a bottleneck in making the circuit in the form of an IC.